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Highly siderophile element depletion in the Moon.

ABSTRACT: Coupled ¹⁸⁷Os/¹⁸⁸Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for Apollo 12 (12005, 12009, 12019, 12022, 12038, 12039, 12040), Apollo 15 (15555) and Apollo 17 (70135) mare basalts, along with mare basalt meteorites La Paz icefield (LAP) 04841 and Miller Range (MIL) 05035. The most magnesian samples have chondrite-relative HSE abundances and chondritic measured and calculated initial ¹⁸⁷Os/¹⁸⁸Os, with mare basalts having consistently low HSE abundances at ~2 ×10^-5 to 2 ×10^-7 the chondritic abundance. The lower and more fractionated HSE compositions of evolved mare basalts can be reproduced with bulk-partition coefficients of ~2 for Os, Ir, Ru, Pt and Pd and ~1.5 for Re. Lunar mare basalt bulk-partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions, leading to increased HSE compatibility. The chondritic-relative abundances and chondritic ¹⁸⁷Os/¹⁸⁸Os of the most primitive high-MgO mare basalts cannot be explained through regolith contamination during emplacement at the lunar surface. Instead, mare basalt compositions can be modelled as representing ~5-11% partial melting of metal-free sources with low Os, Ir, Ru, Pd (~0.1 ng g^-1), Pt (~0.2 ng g^-1) Re (~0.01 ng g^-1) and S, with sulphide-melt partitioning between 1000 and 10000. Apollo 12 olivine-, pigeonite- and ilmenite normative mare basalts define an imprecise ¹⁸⁷Re-¹⁸⁷Os age of 3.0 ±0.6 Ga. This age is within uncertainty of ¹⁴⁷Sm-¹⁴³Nd ages for the same samples and the isochron yields an initial ¹⁸⁷Os/¹⁸⁸Os of 0.109 ±0.008. The Os isotopic composition of the Apollo 12 source indicates that the lunar mantle source of these rocks evolved with Re/Os within ~10% of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted. The similarity in absolute HSE abundances between mare basalts from the Apollo 12, 15 and 17 sites, and from unknown regions of the Moon (La Paz mare basalts, MIL 05035) indicates relatively homogeneous and low HSE abundances within the lunar interior. Low absolute HSE abundances and chondritic Re/Os of mare basalts are consistent with ~0.02% late accretion addition that was added prior to the formation of the lunar crust and significantly prior to cessation of lunar mantle differentiation (>4.4 Ga) to enable efficient mixing and homogenization. The HSE abundances are also consistent with the observed, small ¹⁸²W excess (20 ppm) in the bulk silicate Moon relative to the bulk silicate Earth.

SUBMITTER: Day JMD

PROVIDER: S-EPMC8404368 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as 187Re-187Os and 182Hf-182W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as 187Re-187Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. 182Hf-182W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2-5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of 182Hf-182W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models. The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in 183W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on 182W, the 182W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. The metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.

Project description:Concentrations of highly siderophile elements (HSE) and 187Os/188Os isotopic compositions for eleven impact related rocks from the Apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins. Data for the samples all define linear trends on plots of HSE versus Ir concentrations, whose slopes likely reflect the relative HSE compositions of the dominant impactors that formed the rocks. The inferred Imbrium basin impactor that generated Apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187Os/188Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187Os/188Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages < 3.84 Ga, are within the range of chondritic meteorites, but slightly higher than ratios characterizing previously studied granulitic impactites with reported ages > 4.0 Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ~3.9 Ga, are characterized by suprachondritic 187Os/188Os and HSE/Ir ratios similar to the Apollo 15 impact melt rocks, and may also sample the Imbrium impactor. Three lithic clasts from regolith breccias 60016 and 65095, also with ~3.9 Ga ages, contain multiple impactor components, of which the dominant composition is considerably more suprachondritic than those implicated for Imbrium and Serenitatis (Apollo 17) impactors. The dominant composition recorded in these rocks was most likely inherited from a pre-Imbrium impactor. Consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend. Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar system bodies with bulk chemical compositions that have not yet been sampled by primitive meteorites present in our collections. Alternately, they may reflect the preferential incorporation of evolved metal separated from a fractionated planetesimal core. Pre-3.9 Ga ages for at least some impactor components with both chondritic and fractionated HSE raise the possibility that the bulk of the HSE were added to the lunar crust prior to the later-stage basin-forming impacts, such as Imbrium and Serenitatis, as proposed by Fischer-Gödde and Becker (2012). For this scenario, the later-stage basin-forming impacts were more important with respect to mixing prior impactor components into melt rocks, rather than contributing much to the HSE budgets of the rocks themselves.

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Highly siderophile element depletion in the Moon.

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